You can really work that AAA to maximise your benefits & savings...not so much with the Disney Visa. (You could actually do both, and just use the one that gives the most bang for the buck at the time, too.)
When we booked our May 2008 trip through AAA in January, they were having a "Disney month" special going...besides the 20% room discount, we also got a $25 Disney gift card & a car DVD player. Nice gifts.
And keep in mind, you do get more for your AAA membership than just the opportunity to book Disney trips. You can also book any other vacations through your AAA people, and get additional travel discounts & special offers that way (I know, blasphemy to say there is any other type of vaca!)
Also, you get the road service peace-of-mind...and that's year round, in your home town not just on vaca. And, trip-tic map services customised to your route.
And your AAA card is good for discounts at lots of places, including national chain book stores, florists, restaurants...they'll give you a list, and update it every month.
so that yearly membership fee can easily be made up over the course of the year.